Local regulations, as well as concerns about public safety and liability, require enterprises to provide employees and visitors with an effective means to reach a universal emergency number (e.g., 911 in the U.S., 000 in Australia, and 112 in the European Community) operator in an emergency. The call routes through the local central office, through a switch, to the appropriate Public Safety Answer Point (PSAP), where the call is answered. Each PSAP typically covers one metropolitan area or several or rural counties. At the PSAP, emergency operators determine the nature of the emergency and contact the appropriate agency: typically police, fire, or emergency medical services. A single PSAP is typically responsible for an area covering several independent police and fire departments in the U.S.
With Enhanced 911 (E911), the calling party number, known as an Emergency Location Information Number (ELIN), is sent with the emergency call over Centralized Automatic Message Accounting (CAMA) trunks or via the calling number information element over Integrated Services Digital Network (ISDN) trunks. A suitably configured module at the PSAP uses the ELIN to lookup the caller's documented street address location from the Automatic Location Information (ALI) database. In ISDN, the ELIN is referred to as the location identification number.
To allow operators at the PSAP to call back a caller, IP communication systems use Public Switched Telephone Network (PSTN)-based methods to send an ELIN that identifies the telephone number for an extension from which the emergency call was dialed. If the extension number is not public, premise-based communications systems can be programmed to send an ELIN that is located nearby the calling extension. This is typically done by setting up the Local Area Network (LAN)-based telephony system itself, a media gateway, or a separate server, to automatically send a properly formatted number, such as the public telephone number. To deliver this information to the public emergency services network, the information can be encoded over analog CAMA (which requires an 8-digit ELIN), or over digital ISDN trunks (which require a 10-digit ELIN).
Handling of emergency and crisis alert calls can be complicated when a calling communication device does not have a corresponding extension number. Many enterprises deploy packet-switched and circuit-switched telephones having no personality or number (often referred to as unmerged, x-port, TTI stations or disassociated or unnamed registered stations). There are a number of reasons for using such stations. For example, such station configurations are used in hotelling applications, when an extension is moved from one phone to another (such as through the use of an access code), when a newly installed phone has been registered but not yet assigned an extension, and when administrators do not yet know who will be assigned to the phone (such as a phone in an empty office). During an emergency, a call may be placed by a person using an unnamed station. In the event that emergency personnel attempt to call the person back, such as when the emergency call is disconnected or otherwise terminated prematurely, the switch is unable to direct the call to the calling communication device. Due to the obvious complications from being unable to make an emergency call back, a number of states now require caller identification on 911 calls down to the station level.
Instead of sending the calling number, current switches can send different numbers to the PSAP, when those numbers best represent a user's physical location. Calls to the sent number are forwarded or bridged to the calling number. This capability allows the caller to be called back by responding emergency personnel. However, this function cannot be provided to stations that do not have a number at all, as is the case for unnamed stations.